I’m wondering if FSRS parameters are a model of my ability to memorize anything, that should be given as much data as possible, and is applicable to all decks, or if I should specializing the parameters for individual decks as much as possible?
Yes.
Yes.
No, it is per preset.
Not possible, unless you use different presets. The manual recommends this:
If you have decks that vary wildly in subjective difficulty, it is recommended to assign them separate presets, as their optimal parameters will differ.
You should also read this: Deck Options - Anki Manual.
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Why do I have to use a new preset to pick a different desired retention? Does your brain remember differently at different retention targets? I naively would think the same parameters would be able to be shared between different retention targets.
You don’t. Just change it per deck:
The lower the desired retention, the less you will remember. The higher the desired retention, the more you will remember. But higher desired retention comes at the cost of more reviews and thus more time spent studying. It’s a tradeoff.
But no, your brain doesn’t remember differently just because you change desired retention. The processes involved in remembering stuff still are the same.
Sure. Though apparently math showed that this is only true for decks that are somewhat similar; if they aren’t, maybe you want to create a separate preset.
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That feature was added in 25.09. You’ll need to upgrade to use it.
[And 95% DR is higher than most folks would need. You might want to reconsider that.]
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Ah, thanks, I didn’t realize I was out of date. My package manager has an anki package and an anki-bin package. When I started using anki, anki-bin was the one to use, but it seems now anki is the one that’s up to date.
The main site is the best place to look for the most up-to-date version of the app – Anki - powerful, intelligent flashcards . We can vouch for anything you get through a distro package. Linux Distro Packages - Anki Manual
I’m on NixOS so unfortunately it’s not that simple for me 
If you are using Arch Linux, both packages, anki and anki-bin, were updated to version 25.09 on September 6–7, 2025.
Edit: Sorry, I just noticed that you’re using NixOS.